A typical apparatus for treating foul water and/or waste water includes a storage tank in which contaminated water to be treated is filled, a processing tank for purifying by using a biological method and/or a chemical method the contaminated water supplied from the storage tank, and a sedimentation tank for aggregating and sedimenting suspended solids (SS) contained in the water purified by and discharged from the processing tank such that the SS can be removed.
Since the quality of treated water primarily depends on the sedimentation performance of the sedimentation tank, it is necessary to efficiently design the sedimentation tank.
A well-known conventional sedimentation process is a gravity sedimentation process in which contaminated water is introduced in a sedimentation tank, and heavy SS is sedimented on the floor of the sedimentation tank over time according to the Strokes' Law and then removed. There are many types of sedimentation tanks that can employ such gravity sedimentation process. One of the types is an inclined plate type rectangular sedimentation tank.
FIG. 1 is a side cross-sectional view of a conventional inclined plate type rectangular sedimentation tank 10. The inclined plate type rectangular sedimentation tank 10 includes a hopper portion on an upstream side in which sludge is to be sedimented, and a floor inclination portion on a downstream side such that the (vertical) distance between the bottom of the floor of the sedimentation tank and the surface of water becomes smaller toward the downstream.
A vertical baffle 20 formed of a plurality of inlets 21 is installed in a side portion of the hopper portion so as to be perpendicular to a flow of inflowing water, which enables contaminated water to be introduced in a uniform horizontal flow and at a proper velocity.
A sludge collector 30 in a scrapper type driven by a motor M is installed in the floor inclination portion.
An outflow weir 40 is installed in an upper end portion of the downstream-side wall of the sedimentation tank.
A sludge layer formed by naturally sedimented sludge and by sludge collected by the sludge collector 30 is formed on the hopper portion and the floor inclination portion.
The conventional rectangular sedimentation tank, however, has problems. For example, it requires long sedimentation retention time to treat water with high SS density. Also, if a stream of water is deflected due to, e.g., water temperature fluctuation, sludge sedimented on the floor of the sedimentation tank may be surged to the downstream portion, thereby increasing the sludge layer toward the downstream. In addition, if the sludge is distributed on the floor of the sedimentation tank in a wide and long manner, a density of the sludge sedimented in the hopper portion becomes relatively reduced. As a consequence, the density of a return sludge may not be maintained, which may cause a problem that a microorganism density MLSS of a biological processing tank cannot be maintained.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.